1. Field of the Invention
The present invention relates generally to a fuel injection system for an internal combustion engine, and more specifically, to the fuel injection system, wherein a portion of intake air upstream of a throttle valve is introduced, bypassing the throttle valve, to an area where fuel is injected from a fuel injection valve, so as to facilitate atomization of the injected fuel for reducing harmful components involved in exhaust gas.
2. Description of the Prior Art
In a fuel injection system which injects fuel into an induction system of an internal combustion engine via a fuel injection valve so as to control an operating condition of the engine, it has been strongly demanded that atomization of the injected fuel be facilitated so as to reduce injurious components involved in exhaust gas which are otherwise increased due to a deterioration of combustion in the engine. In order to satisfy this demand, there have been proposed fuel injection systems as disclosed, such as, in Japanese Second (examined) Patent Publication No. 57-54624 and Japanese First (unexamined) Utility Model Publication No. 58-162262.
In either of such disclosed systems, the upstream side of a throttle valve is connected to the vicinity of an injection hole of a fuel injection valve via an air duct with an air control valve disposed therein, bypassing the throttle valve. With this structure, when effective atomization of fuel injected from the injection hole of the fuel injection valve is not expected, such as, during an engine idling, the air control valve is controlled to open and close substantially in synchronism with the fuel injection. Accordingly, during the engine idling, a portion of the intake air upstream of the throttle valve, which is close to the atmospheric pressure, is introduced to the neighborhood of the fuel injection hole via the air duct for a time period substantially synchronous with the fuel injection by means of the opening and closing operation of the air control valve so as to facilitate the atomization of the injected fuel. Since the supply of the intake air via the air duct is limited to the time period of the fuel injection, the atomization of the injected fuel is effected to a sufficient level with an idling engine speed being prevented from increasing in comparison with a continuous supply of the intake air via the air duct.
In either of the foregoing conventional fuel injection systems, however, since a known idling speed control valve should be separately provided for adjusting the idling engine speed, the entire system inevitably becomes complicated. Specifically, the system performs the control of the opening and closing operation of the air control valve substantially in synchronism with that of the fuel injection valve in addition to an opening degree control of the known idling speed control valve. This may lead to an increased manufacturing cost of the system.
On the other hand, another fuel injection system has been proposed as disclosed in, such as, Japanese First (unexamined) Patent Publication No. 58-206851. Intake air on the upstream side of a throttle valve is introduced into a serge tank as well as to the vicinity of a fuel injection hole of a fuel injection valve via an air duct with an air control valve disposed therein. A supply amount of the intake air via the air duct is adjusted by a duty-cycle control which controls an opening degree of the air control valve.
As appreciated, the supply of the intake air from the upstream side of the throttle valve to the vicinity of the fuel injection hole via the air duct is realized by means of a pressure differential between an intake vacuum generated in an intake port of the engine and a pressure close to the atmospheric pressure in the air duct. Accordingly, in order to ensure a sufficiently high level of the pressure differential, a sectional open area of the air duct should be set to a value which is at least about two to three times as large as a sum of a sectional open area of each air injection hole or a total air injecting area through which the intake air is injected to the neighborhood of the fuel injection hole.
In the foregoing conventional fuel injection system as disclosed in Japanese First (unexamined) Patent Publication No. 58-206851, however, since the opening degree of the air control valve in the air duct is controlled as described above, a pressure loss or drop is caused when the intake air flows through the air control valve, resulting in a reduction of the above-noted pressure differential. This, in turn, causes dropping of a flow velocity of the intake air injected from each air injecting hole, leading to a likelihood that the atomization of the injected fuel is not realized to a sufficient level.
Further, in known fuel injection systems, including those as described above, when the throttle valve is substantially closed, i.e. under a low engine load, a pressure generated on the downstream side of the air control valve largely fluctuates due to the opening and closing operation of the air control valve, which produces a pulsation noise of a high level in synchronism with the pressure fluctuation to increase a noise in a passenger's compartment. In order to solve this problem, it may be one option to provide, for example, a known resonator as having been employed for reducing a pulsation noise when, such as, controlling the idling engine speed with the idling speed control valve. However, as appreciated, since the resonator utilizes the resonance principle, a suppressible pulsation noise is limited to a specific frequency band. Accordingly, when a frequency of the pulsation noise largely varies depending on an engine speed as in the above-noted case, the provision of the resonator is hardly effective.